The application of image-guided systems with or with-out support by surgical robots relies on the accuracy of the naviga-tion process, including patient-to-image registration. The surgeon must carry out the procedure based on the information provided by the navigation system, usually without being able to verify its cor-rectness beyond visual inspection. Misleading surrogate parame-ters such as the fiducial registration error are often used to describe the success of the registration process, while a lack of methods de-scribing the effects of navigation errors, such as those caused by tracking or calibration, may prevent the application of image guid-ance in certain accuracy-critical interventions. During minimally invasive mastoidectomy for cochlear implantation, a direct tunnel is drilled from the outside of the mastoid to a target on the cochlea based on registration using landmarks solely on the surface of the skull. Using this methodology, it is impossible to detect if the drill is advancing in the correct direction and that injury of the facial nerve will be avoided. To overcome this problem, a tool localization method based on drilling process information is proposed. The al-gorithm estimates the pose of a robot-guided surgical tool during a drilling task based on the correlation of the observed axial drilling force and the heterogeneous bone density in the mastoid extracted from 3-D image data. We present here one possible implementa-tion of this method tested on ten tunnels drilled into three human cadaver specimens where an average tool localization accuracy of 0.29 mm was observed.